Such a pressure-relief valve that is mounted in a port on the top of a transformer casing is known from U.S. Pat. No. 3,914,528. It has a circular housing flange that is screwed to the port of the transformer casing. On the lower face of the housing flange turned toward the top of the transformer casing there is an annular seal creating an oil-tight seal. The port of the housing flange is normally closed by a spring-loaded valve body described further below. A round downwardly open cup-shaped housing cover is secured on the housing flange by bolts and screws. The upper nearly horizontal face of the housing cover has an inner concentric step. Between the housing cover and the above-described valve body there are one or more prestressed compression springs that are braced upwardly against the inner face of the housing cover and downwardly against the top face of the valve body so as to push same down against the housing flange such that its port is closed oil-tight. To this end the upper side of the housing flange turned toward the valve body has an annular seal ring. If the pressure inside the transformer casing exceeds the response pressure of the springs, the valve body lifts and allows pressure equalization; subsequently it is pushed by the springs again back down on the annular seal on the upper side of the housing flange. In addition it is possible with this known pressure-relief valve to provide on the upper face, outside the housing cover, optical indicators that can be seen from afar to indicate if the pressure-relief valve has opened. Finally there are outside the housing cover also electrical monitor contacts or switches that serve for remote monitoring or indication as well as to turn off the transformer.
A further highly similar pressure-relief valve is known from U.S. Pat. No. 4,843,187. It has a particular cross section and special mounting means for the above-described peripheral seal ring between the upper face of the housing flange and the lower face of the valve body.
Another pressure-relief valve is known from U.S. Pat. No. 4,676,266. Here the valve cover is formed as a cup whose downwardly open side walls fit with another seal that is set in the annular face of the housing flange. This additional seal has a flat cross-sectional shape; when closed it is bent to the side—like a windshield-wiper blade—and seals the valve body at its side walls additionally against the housing flange. In case of a problem, when the valve plate rises up because of an overpressure in the transformer casing, this pressure is effective against the entire face of the valve body; the force effective on the body thus is greater and the valve snaps open very fast, inside a few milliseconds.
All these pressure-relief valves have several disadvantages. The main one is that if there is a problem with excessive pressure in the transformer casing, when the valve body is open, the highly pressurized medium is not under control and can squirt out of the pressure-relief valve. This medium, which squirts out abruptly and without warning, is extremely hot so that it poses a substantial risk of burns for nearby personnel, not to mention fouling the surroundings. A further disadvantage with the known pressure relief valves is that the switch contacts are provided without protection outside the actual device, with no shielding and not protected from UV-rays, ozone, rain, and the like. In addition they and their cables are exposed to the hot oil released when the valve opens.
A shield for pressure-relief valves is known from WO 1998/054498 (U.S. Pat. No. 5,987,893) that is fitted over the valves and that has a relief port that ensures that any released oil is guided away when the valve opens. Even this solution has technical problems. To start with it is a pure retrofit. The shield is mounted on the pressure-relief valve by means of the existing bolts and screws that secure the housing cover on the housing flange with the highly prestressed springs between them. To install the known shield, these nuts must be loosened, thereby releasing the spring-loaded housing cover. This runs the risk that the springs will pop up the housing cover and injure the installer. In another variant the known shield is secured by an additional flange adapter with the transformer casing. This is an expensive variant that not only requires extra parts, but that also creates seal problems which are dealt with by the provision of further seals that further complicate the assembly.